It is testing and integration with system packages available on the distro that costs most and for 3.4 releases the Ubuntu/Debian packaging was still using the Rube-Goldberg-machine-like go-oo-packaging. We finally got rid of that with 3.5 (which was a lot of work in itself). Also note that there are packages avaliable for 3.5 in the ppa. It will take some time to backport these to older releases though.
You are most welcome to help out!
Bjoern Michaelsen – Ubuntu LibreOffice maintainer
P.S.: Sometimes backporting itself is not possible without loosing functionality, e.g. when a new version of LibreOffice needs a new version of some other package (e.g. PostgeSQL) - providing the backported update would mean to silently drop certain features that were available before as the external package is not in the ppa. That would not be acceptable to most users.
Work on Ubuntu/Debians packaging can be followed here: